Ovulation Calculator
Find your ovulation date, fertile window, and next expected period — calculated instantly from your last menstrual period and cycle length. View the next 3 cycles at a glance.
All calculations happen in your browser. No data is sent anywhere.
Calculate Your Ovulation Date
Enter the first day of your last period and your average cycle length to find your ovulation date and fertile window.
Your Results
| Last menstrual period | — |
|---|---|
| Next expected period | — |
| Ovulation date | — |
| Fertile window start | — |
| Fertile window end | — |
| Luteal phase | — |
Next 3 Cycles
Projected ovulation and fertile window for your next three menstrual cycles.
| Cycle | Period Start | Ovulation | Fertile Window |
|---|
Dates are estimates. Actual timing varies based on individual factors.
Understanding Ovulation & Fertility
How ovulation works
Ovulation is the release of a mature egg from one of the ovaries. It happens once per menstrual cycle, typically around 14 days before your next period begins — regardless of your total cycle length.
The cycle is driven by hormones: FSH stimulates a follicle to grow, rising estrogen triggers an LH surge, and the surge causes the follicle to release the egg within 24–36 hours.
- Ovulation = LMP + (cycle length − 14) days
- The egg survives 12–24 hours after release
- Basal body temperature rises slightly after ovulation
Fertile window explained
The fertile window is the time each cycle when intercourse can lead to pregnancy. It spans 7 days because sperm can live inside the reproductive tract for up to 5 days.
- Starts 5 days before ovulation
- Includes the day of ovulation
- Ends 1 day after ovulation (7 days total)
- Peak fertility: 1–2 days before ovulation
Having intercourse in the 2–3 days leading up to ovulation gives the highest chance of conception.
Signs of ovulation
Your body gives several signals around ovulation that you can learn to recognize:
- Cervical mucus: Becomes clear, slippery, and stretchy (like raw egg whites)
- Basal body temperature: Rises 0.2–0.5°C after ovulation
- Mittelschmerz: Mild one-sided pelvic cramping or pain
- LH surge: Detected by ovulation predictor kits 24–36 h before ovulation
- Breast tenderness: Light soreness due to hormonal shifts
Factors affecting your cycle
Many factors can shift ovulation timing and cycle length, making calendar-based predictions less precise:
- Stress: Can suppress or delay ovulation
- Illness or travel: May disrupt hormonal signals
- Significant weight change: Affects estrogen and cycle regularity
- Recent hormonal contraception: Can delay return of regular ovulation
- Thyroid conditions and PCOS: Often cause irregular cycles
If your cycles vary significantly month to month, tracking basal body temperature and cervical mucus alongside this calculator improves accuracy.
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Calculate age →Frequently Asked Questions
How do you calculate ovulation date?
Ovulation typically occurs about 14 days before your next period. The formula is: Ovulation Date = First day of last menstrual period + (cycle length − 14) days. For a standard 28-day cycle, ovulation falls around day 14. For a 30-day cycle it falls around day 16, and for a 35-day cycle around day 21. This calculator applies that formula automatically based on the cycle length you enter.
What is the fertile window?
The fertile window is the span of days in your cycle when pregnancy is possible. It spans 7 days: the 5 days before ovulation, the day of ovulation itself, and 1 day after ovulation. Sperm can survive in the reproductive tract for up to 5 days, which is why the window extends before ovulation. The egg can only be fertilized for 12–24 hours after it is released. Having intercourse in the 2–3 days immediately before ovulation gives the highest probability of conception.
How long does ovulation last?
The ovulation event itself — when the egg is released from the follicle — lasts only 12 to 24 hours. The egg must be fertilized within that window. However, because sperm can survive for up to 5 days inside the reproductive tract, the practical fertile window extends to about 7 days total. This is why timing intercourse in the days leading up to ovulation is an effective strategy for conception.
What are the signs of ovulation?
Common signs include: cervical mucus that becomes clear, slippery, and stretchy (like raw egg whites); a slight rise in basal body temperature (BBT) of about 0.2–0.5°C after ovulation; mild one-sided pelvic pain called mittelschmerz; light spotting; breast tenderness; and increased sex drive. Ovulation predictor kits (OPKs) detect the LH surge that triggers ovulation, typically 24–36 hours before it occurs, giving you advance notice of your most fertile days.
Can cycle length affect ovulation timing?
Yes. The luteal phase — the time from ovulation to the next period — is relatively fixed at about 14 days for most people. It is the follicular phase (from the start of the period to ovulation) that varies with cycle length. A shorter cycle means ovulation happens earlier; a longer cycle means it happens later. For example, in a 24-day cycle ovulation occurs around day 10, while in a 35-day cycle it occurs around day 21. People with irregular cycles may find it harder to predict ovulation by calendar alone and may benefit from tracking additional signs or using OPKs.